1. Field of the Invention
The invention relates to a resampling detection apparatus capable of detecting whether a digital input signal such as a video signal or audio signal is resampled or not, and a resampling apparatus using the same.
2. Related Art
Resampling process for converting a sampling frequency is often applied to a digital signal generated by sampling a video signal. The resampling process includes downsampling (subsampling) for generating new sampling data by decimating the original sampling data, and upsampling for generating new sampling data by interpolating the original sampling data. Resampling process is also called sampling rate conversion process.
For example, a resampling apparatus incorporated in a recording and reproducing apparatus such as a digital VTR processes an input video signal by downsampling with a decimating filter for recording, and processes the video signal being read out from the recording medium by upsampling by interpolating filter reversely for reproducing. When dubbing is repeated by using such resampling apparatus, however, filtering is processed by the resampling apparatus on every dubbing, and downsampling and upsampling are repeated. As a result, the frequency characteristic of the signal deteriorates.
Various methods have been proposed so far to solve these problems. For example, in downsampling process, when it is detected that the input signal is a signal which has already been upsampled, that is, the input signal is a signal which is reproduced after being recorded, the sample is extracted by skipping the filtering process. Furthermore, the upsampling process includes a method which causes data of undecimated samples not to be changed before and after interpolating process. According to such a method, deterioration of a signal by repetition of downsampling and upsampling can be prevented (see, for example, patent document 1).
FIGS. 10A to 10C show sample phases of one-dimensional signal converted by a conventional resampling apparatus. FIG. 10A shows downsampling, and FIG. 10B shows upsampling. In the diagram, the black circle “•” shows a sample decimated by downsampling, the white circle “∘” shows a sample directly output, and the double circle “⊚” shows a sample interpolated by upsampling.
As shown in FIG. 10A, when recording input signal a while downsampling it, in input signal a of sampling rate Fs, every other sample (•) of input signal a is decimated, and the remaining samples (∘) are output directly. As a result, decimated signal b of sampling rate Fs/2 is obtained, and it is recorded in the recording medium.
Further, as shown in FIG. 10B, when upsampling on reproduction of a signal from the recording medium, a decimated signal b of sampling rate Fs/2 is received, and in addition to each sample (∘) of input signal b, new sample (⊚) of data obtained by interpolation is inserted in intermediate position of each sample (∘). Thus, the interpolated signal c of sampling rate Fs is obtained, and it is reproduced and output. Afterwards, when downsampling and recording this signal c, the signal b can be restored and recorded, by decimating the sample (⊚) inserted on the upsampling. Therefore, when downsampling and upsampling are repeated by this resampling apparatus, undecimated sample (∘) is always held, and frequency characteristic of the signal does not deteriorate.
Patent document: JP, 04-185127, A
However, the conventional resampling apparatus cannot be applied in the case where sample phases are not matched before and after resampling process such as downsampling process or upsampling process. In recent sampling process, phases are often not matched before and after resampling process, and the conventional resampling apparatus cannot be applied in such a case.
For example, using two-tap filter, as shown in FIG. 10C, when a signal d of sampling rate of 48 kHz (signal values: d0, d1, d2, d3) is downsampled to a signal e of sampling rate of 24 kHz (signal values: e0, e1, e2) without matching in phase, for example, a signal e0 is expressed in formula (1) and a signal e1 in formula (2).
                              e          0                =                                            d              0                        +                          d              1                                2                                    (        1        )                                          e          1                =                                            d              1                        +                          d              2                                2                                    (        2        )            
Further, when a signal e is upsampled to a signal d′ of sampling rate of 48 kHz (signal values: d′0, d′1, d′2, d′3), for example, a signal d′1 is expressed in formula (3), and does not coincide with the original signal d1.
                              d          1          ′                =                                            d              0                        +                          2              ⁢                              d                1                                      +                          d              2                                2                                    (        3        )            
Thus, when sampling process not coinciding in phase is repeated, a signal deteriorates after every repetition. This deterioration cannot be suppressed by the conventional resampling apparatus.